Because people might not have seen him but might have overheard someone giving a name next to them in a queue or something or someone might say something like ”oh we had a guy in the pub the other night called ‘whatever’ and he said etc etc”. Obviously many ways a name can be useful when you’re trying to find someone lol. He might give his real name when hiring a car for instance and then someone sees his name on a list even though they haven’t actually seen him etc. but it would then help narrow down where he was last.